Integrated wood preserving processes comprising the artificial drying of green wood products and the subsequent impregnation of the dried wood products with a preservative agent under pressure are old and well-known. One such known integrated process utilizes a boiling under vacuum step to dry the wood products and a pressure step to thereafter force preservative agent into the cells of the dried wood products. This process has particular applicability to the drying and impregnation of wood products such as bridge ties, mine ties, railroad crossties and switch ties, pilings for both land and water use, utility poles and the like. In this process, coal tar based creosote-coal tar and creosote-petroleum oil mixed solutions containing relatively volatile naphthalene fractions are utilized in both the drying and pressure steps.
In utilizing the above integrated process for drying and impregnating wood products with one of the aforementioned naphthalene rich preservative agents, problems can and do occur. These include, for example, the plugging of process equipment and the contamination of process derived waste waters with naphthalene as a result of the volatile naphthalene fractions in the preservative being stripped from the wood products during the drying step. These problems further are compounded by the fact that in most commercial operations, it is usual practice to recover this naphthalene and return it to the preservative agent storage or supply tanks. Thus, with each succeeding use of the preservative agent from the storage or supply tanks, the same naphthalene not only is being restripped from the preservative agent during each drying phase, but also this naphthalene is building up in concentration in the recycling preservative agent. Such build-up only further contributes to the aforementioned process equipment plugging and waste water contamination problems.